Thermoviscoplastic modelling of asymmetric effects for polymers at large strains

AbstractGlassy polymers such as polycarbonate exhibit different behaviours in different loading scenarios, such as tension and compression. To this end a flow rule is postulated within a thermodynamic consistent framework in a mixed variant formulation and decomposed into a sum of weighted stress mode related quantities. The different stress modes are chosen such that they are accessible to individual examination in the laboratory, where tension and compression are typical examples. The characterisation of the stress modes is obtained in the octahedral plane of the deviatoric stress space in terms of the Lode angle, such that stress mode dependent scalar weighting functions can be constructed. Furthermore the numerical implementation of the constitutive equations into a finite element program is briefly described. In a numerical example, the model is used to simulate the laser transmission welding process

The Error and Repair Catastrophes: A Two-Dimensional Phase Diagram in the Quasispecies Model

This paper develops a two gene, single fitness peak model for determining the equilibrium distribution of genotypes in a unicellular population which is capable of genetic damage repair. The first gene, denoted by $\sigma_{via}$, yields a viable organism with first order growth rate constant $k > 1$ if it is equal to some target ``master'' sequence $\sigma_{via, 0}$. The second gene, denoted by $\sigma_{rep}$, yields an organism capable of genetic repair if it is equal to some target ``master'' sequence $\sigma_{rep, 0}$. This model is analytically solvable in the limit of infinite sequence length, and gives an equilibrium distribution which depends on \mu \equiv L\eps , the product of sequence length and per base pair replication error probability, and \eps_r , the probability of repair failure per base pair. The equilibrium distribution is shown to exist in one of three possible ``phases.'' In the first phase, the population is localized about the viability and repairing master sequences. As \eps_r exceeds the fraction of deleterious mutations, the population undergoes a ``repair'' catastrophe, in which the equilibrium distribution is still localized about the viability master sequence, but is spread ergodically over the sequence subspace defined by the repair gene. Below the repair catastrophe, the distribution undergoes the error catastrophe when $\mu$ exceeds \ln k/\eps_r , while above the repair catastrophe, the distribution undergoes the error catastrophe when $\mu$ exceeds $\ln k/f_{del}$, where $f_{del}$ denotes the fraction of deleterious mutations.Comment: 14 pages, 3 figures. Submitted to Physical Review

Nanoscale grains, high irreversibility field, and large critical current density as a function of high energy ball milling time in C-doped magnesium diboride

Magnesium diboride (MgB2) powder was mechanically alloyed by high energy ball milling with C to a composition of Mg(B0.95C0.05)2 and then sintered at 1000 C in a hot isostatic press. Milling times varied from 1 minute to 3000 minutes. Full C incorporation required only 30-60 min of milling. Grain size of sintered samples decreased with increased milling time to less than 30 nm for 20-50 hrs of milling. Milling had a weak detrimental effect on connectivity. Strong irreversibility field (H*) increase (from 13.3 T to 17.2 T at 4.2 K) due to increased milling time was observed and correlated linearly with inverse grain size (1/d). As a result, high field Jc benefited greatly from lengthy powder milling. Jc(8 T, 4.2 K) peaked at > 80,000 A/cm2 with 1200 min of milling compared with only ~ 26,000 A/cm2 for 60 min of milling. This non-compositional performance increase is attributed to grain refinement of the unsintered powder by milling, and to the probable suppression of grain growth by milling-induced MgO nano-dispersions.Comment: 12 pages, 11 figure

Comment on "Antilocalization in a 2D Electron Gas in a Random Magnetic Field"

In a recent Letter, Taras-Semchuk and Efetov reconsider the problem of electron localization in a random magnetic field in two dimensions. They claim that due to the long-range nature of the vector potential correlations an additional term appears in the effective field theory ($\sigma$-model) of the problem, leading to delocalization at the one-loop level. This calls into question the results of earlier analytical studies, where the random magnetic field problem was mapped onto the conventional unitary-class $\sigma$-model, implying that the leading quantum correction is of two-loop order and of a localizing nature. We show in this Comment, however, that the new term in fact does not exist and was erroneously obtained by Taras-Semchuk and Efetov because of an inconsistent treatment violating gauge invariance.Comment: 1 page, 2 figure

Effects of Neutron Irradiation on Carbon Doped MgB2 Wire Segments

We have studied the evolution of superconducting and normal state properties of neutron irradiated Mg(B$_{.962}$C$_{.038}$)$_2$ wire segments as a function of post exposure annealing time and temperature. The initial fluence fully suppressed superconductivity and resulted in an anisotropic expansion of the unit cell. Superconductivity was restored by post-exposure annealing. The upper critical field, H$_{c2}$(T=0), approximately scales with T$_c$ starting with an undamaged T$_c$ near 37 K and H$_{c2}$(T=0) near 32 T. Up to an annealing temperature of 400 $^ o$C the recovery of T$_c$ tends to coincide with a decrease in the normal state resistivity and a systematic recovery of the lattice parameters. Above 400 $^ o$C a decrease in order along the c- direction coincides with an increase in resistivity, but no apparent change in the evolution of T$_c$ and H$_{c2}$. To first order, it appears that carbon doping and neutron damaging effect the superconducting properties of MgB$_2$ independently

Potential Energy Surface for H_2 Dissociation over Pd(100)

The potential energy surface (PES) of dissociative adsorption of H_2 on Pd(100) is investigated using density functional theory and the full-potential linear augmented plane wave (FP-LAPW) method. Several dissociation pathways are identified which have a vanishing energy barrier. A pronounced dependence of the potential energy on ``cartwheel'' rotations of the molecular axis is found. The calculated PES shows no indication of the presence of a precursor state in front of the surface. Both results indicate that steering effects determine the observed decrease of the sticking coefficient at low energies of the H_2 molecules. We show that the topology of the PES is related to the dependence of the covalent H(s)-Pd(d) interactions on the orientation of the H_2 molecule.Comment: RevTeX, 8 pages, 5 figures in uufiles forma

Local Isoelectronic Reactivity of Solid Surfaces

The quantity w^N(r) = ( 1/ k^2 T_el)[partial n(r, T_el) / partial T_el]_(v(r),N) is introduced as a convenient measure of the local isoelectronic reactivity of surfaces. It characterizes the local polarizability of the surface and it can be calculated easily. The quantity w^N(r) supplements the charge transfer reactivity measured e.g. by the local softness to which it is closely related. We demonstrate the applicability and virtues of the function w^N(r) for the example of hydrogen dissociation and adsorption on Pd(100).Comment: RevTeX, 13 pages, 3 figures, to appear in Phys. Rev. Let

Status and distribution of colonial waterbirds in coastal Virginia: 2018 breeding season

Colonial waterbirds are highly visible components of coastal avifaunas that share the unusual characteristic of nesting in dense assemblages. One consequence of having large portions of populations nesting in few locations is that even restricted disturbance may have profound consequences on a population level. Development of conservation strategies for these sensitive species requires current status and distribution information. In the fall of 1992, a consortium of agencies and individuals agreed that a comprehensive monitoring program for the Virginia colonial waterbird community was needed and that assessments should be made on regular (initially every 10 years but reduced to 5 years in 2003) intervals for trend analyses. Systematic surveys have been conducted during the breeding seasons of 1993, 2003, 2008 and 2013. The 2018 survey reported here is the fifth in the time series. These surveys have covered colonial waterbird populations (24 species – Great Blue Herons were not included in 2008 and 2018 due to budgetary constraints) throughout the Coastal Plain province of Virginia. We surveyed 270 waterbird colonies during the breeding season of 2018. Colonies supported an estimated 43,159 breeding pairs of 23 species. Gulls were the most abundant group with more than 19,700 breeding pairs. Terns and waders accounted for 7,129 and 6,386 pairs respectively. Although they have declined dramatically, Laughing Gulls continue to be the most abundant species and were three times more abundant than any other species, accounting for nearly 40% of the total waterbird community. The barrier island/lagoon system of the Eastern Shore was the most important region for the majority of colonial species encountered. In 2018, this region supported 22 of the 23 species evaluated. The Eastern Shore accounted for 50.5% and 46.6% of all breeding pairs and colonies respectively. For 17 of the 23 species, the region supported more than 50% of the known coastal population. The colonial waterbird community as a whole in coastal Virginia has declined dramatically since 1993 (2018 survey did not include Great Blue Herons or all Great Egrets). Population estimates for 15 (68%) of the 22 species assessed declined between 1993 and 2018. Declines varied considerably between species with 14 species declining more than 40% and 9 species declining more than 60%. Cattle Egrets showed the highest loss rate (-96.7%), declining from an estimated 1,459 to only 48 pairs. Little Blue Herons declined by 83% from 374 to only 64 pairs. Seven species increased between 1993 and 2018. Dramatic expansions were documented for White Ibis, Double-crested Cormorant, and Brown Pelican. Over the past 25 years, two major forces appear to be shaping the colonial waterbird community in Virginia. These include 1) regional shifts in population centers that are driving population increases in Virginia and 2) habitat degradation related to sea-level rise. With the exception of Great Egrets, all species that have increased over the past 20 years have experienced ongoing range expansions and are riding a population wave that is progressing through Virginia. This includes Great Black-backed Gull, Double-crested Cormorant, Brown Pelican, and White Ibis. Most of the decline in medium-sized waders is being driven by habitat loss related to erosion of islands. This erosion results from sea-level rise, is ongoing and represents a significant threat to these populations. Several ground-nesting seabirds are likely more directly impacted by the loss of viable habitat and demographic impacts related to frequent flooding. The most notable example is the Laughing Gull that has experienced a catastrophic decline in both population and distribution